The usefulness of packet switching networks for the transmission of digital information, particularly burst type information, has long been recognized. Such networks are generally point-to-point in nature in that a packet from a single source is directed to a single destination by an address attached to the packet. The network responds to the packet address by connecting the packet to the appropriate destination.
Packet switching networks are also used which combine burst type data with the more continuous types of information such as voice, high quality audio, and motion video. Commercialization of voice, video and audio transmission makes it desirable to be able to connect packets to multiple destinations, called packet broadcasting. For example, a broadcast video service such as pay-per-view television involves a single source of video packets, each of which is directed to multiple video receivers. Similarly, conferencing capabilities for voice communication also require single source to multiple destination transmission.
One prior packet broadcast arrangement such as that shown in J. S. Turner, U.S. Pat. No. 4,734,907, Mar. 29, 1988, comprises a network consisting of a packet duplication arrangement followed by a packet routing arrangement. As a broadcast packet enters this network, packet copies are made in the packet duplicating arrangement until as many copies exist as there are destinations for the packet. A translation table look up is then performed at the duplication arrangement outputs for each of the packet copies to provide a different, single destination address for each copy. All of the packet copies with their new packet addresses are then applied to the packet routing arrangement, which connects them to the appropriate network output ports.
Problems exist with this prior packet broadcast arrangement. First, a separate packet duplication arrangement must be provided prior to the routing arrangement which adds to the expense and complexity of the network and which sends multiple packet copies to the inputs of the routing arrangement. Sending multiple copies to the routing network inputs (as opposed to sending a single packet which is duplicated by the routing arrangement) increases the total bandwidth which must be provided within the routing arrangement. Another problem with this prior network is that each of the duplication arrangement output ports requires a translation table which must be consulted each time a broadcast packet occurs and must be frequently updated. Throughout a large network, maintaining the consistency of the translation table data, especially when packet destinations are added or deleted, is complex.
Another prior packet broadcast arrangement is described in M. N. Ransom et al., U.S. Pat. No. 4,701,906, issued October 20, 1987. The Ransom et al., arrangement affixes to each incoming packet an address which is capable of directing the packet to multiple, but not all combinations of network destinations. This network avoids the use of a separate duplication arrangement but many combinations of outputs cannot be reached by incoming packets. This problem limits the usefulness of the network for commercial applications.